			var canvas=document.getElementById("myCanvas");
			var ctx=canvas.getContext("2d");
			var windowX=document.documentElement.clientWidth;
			var windowY=document.documentElement.clientHeight;
			canvas.width=windowX;
			canvas.height=windowY;
			var bgImage=new Image();
			bgImage.src="img/background_2.png";
			bgImage.onload=function(){
				//控制图片移动的位置
				var picY=0;
				//写一个用来绘制图片的函数
				function drawBGImage(){
					ctx.clearRect(0,0,canvas.width,canvas.height);
					picY+=2;
					ctx.drawImage(bgImage,0,0,bgImage.width,bgImage.height,0,picY,canvas.width,canvas.height);
					ctx.drawImage(bgImage,0,0,bgImage.width,bgImage.height,0,picY-canvas.height,canvas.width,canvas.height);
					//如果picY超过了canvas的尺寸范围，让picY回到0位
					if(picY>=canvas.height){
						picY=0;
					}
					window.requestAnimationFrame(drawBGImage);
				}
				drawBGImage();
			}